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The Aarhus Chamber Campaign on Highly Oxidized Multifunctional Organic Molecules and Aerosols (ACCHA): Particle Formation and Detailed Chemical Composition at Different Temperatures Kasper Kristensen1, Louise N. Jensen2, Lauriane. L. J. Quéléver3, Sigurd Christiansen2, Bernadette Rosati2,4, Jonas Elm2, Ricky Teiwes4, Henrik B. Pedersen4, MarianneGlasius2, Mikael Ehn3, Merete Bilde2 5 1Department of Engineering, Aarhus University, 8000 Aarhus C, Denmark 2Department of Chemistry and iClimate, Aarhus University, 8000 Aarhus C, Denmark 3Institute for Atmospheric and Earth System Research – INAR / Physics, P.O. Box 64, FI-00014, University of Helsinki, Finland 4Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark 10
Correspondence to: Kasper Kristensen ([email protected]) and Merete Bilde ([email protected])
Abstract
Little is known about the effects of low temperatures on the formation of SOA from α-pinene. In the current work, ozone-
initiated oxidation of α-pinene at initial concentrations of 10 and 50 ppb, respectively, is performed at temperatures of 20, 0
and -15 °C in the Aarhus University Research on Aerosol (AURA) smog chamber during the Aarhus Chamber Campaign on 15
highly oxidized multifunctional organic molecules and Aerosol (ACCHA). Here, we show how temperature influences the
formation and chemical composition of α-pinene-derived SOA with a specific focus on the formation of organic acids and
dimer esters. With respect to particle formation, results show significant increase in both particle formation rates, particle number
concentrations and particle mass concentrations at lower temperatures. In particular, the number concentrations of sub-10 nm particles
were significantly enhanced at the lower 0 and -15 °C temperatures. Temperatures also affect chemical composition of the formed 20
SOA. Here, detailed off-line chemical analyses show organic acids contributing from 15 to 30 % by mass, with highest
contributions observed at the lower temperatures indicative of enhanced condensation of these semi-volatile species. In
comparison, 30 identified dimer esters contribute between 4 – 11 % to SOA mass. No significant differences in the chemical
composition (i.e. organic acids and dimer esters) of the α-pinene-derived SOA particles are observed between experiments
performed at 10 and 50 ppb initial α-pinene concentrations, thus suggesting a higher influence of reaction temperature 25
compared to that of α-pinene loading on the SOA chemical composition. Interestingly, the effect of temperature on the
formation of dimer esters differs between the individual species. The formation of less oxidized (oxygen-to-carbon ratio (O:C)
< 0.4) dimer esters is shown to increase at lower temperatures while the formation of the more oxidized (O:C > 0.4) species is
suppressed, consequently resulting in temperature-modulated composition of the α-pinene derived SOA. Temperature ramping
experiments exposing α-pinene-derived SOA to changing temperatures (heating and cooling) reveal that the chemical 30
composition of the SOA with respect to dimer esters is governed almost solely by the temperature during the initial oxidization
and insusceptible to subsequent changes in temperature. Similarly, the resulting SOA mass concentrations were found to be
more influenced by the initial α-pinene oxidation temperatures, thus suggesting that the formation conditions to a large extent
govern the type of SOA formed, rather than the conditions to which the SOA is later exposed.
For the first time, we discuss the relation between the identified dimer ester and the highly oxidized multifunctional organic 35
molecules (HOMs) measured by Chemical Ionization Atmospheric Pressure interface Time-of-Flight mass spectrometer (CI-APi-
TOF) during ACCHA experiments. We propose that, although very different in chemical structures and O:C-ratios, dimer
esters and HOMs may be linked through the mechanism of RO2 autoxidation, and that dimer esters and HOMs merely represent
two different fates of the RO2 radicals.
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
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1. Introduction 40
The oxidation of volatile organic compounds (VOC) constitutes an important source of secondary organic aerosol in the
atmosphere. Due to its atmospheric reactivity and its high estimated global emission rate of ~30 Tg/yr (Sindelarova et al.,
2014), the atmospheric oxidation of the biogenic VOC α-pinene has been widely studied. The boreal forests are considered
abundant sources of α-pinene with varying but sizeable emissions occurring all year around (Hakola et al., 2003;Hakola et al.,
2009;Noe et al., 2012). For this reason, the oxidation of α-pinene and subsequent formation of secondary organic aerosol 45
(SOA) is expected to occur at conditions with a wide range of VOC concentrations and atmospheric temperatures.
A number of smog chamber studies have investigated the effect of α-pinene concentration and reaction temperature on the
formation of SOA from α-pinene oxidation (Svendby et al., 2008;Pathak et al., 2007;Tillmann et al., 2010;Warren et al.,
2009;Kourtchev et al., 2016b;Kristensen et al., 2017). Most of the published studies, however, focus on SOA mass yield
(defined as mass of SOA formed per mass of reacted VOC) and reports higher yields with increasing VOC concentrations and 50
lower temperatures. The effect of subzero temperatures, on gas phase oxidation products, nucleation, particle growth and
particle chemical composition, remains a largely unexplored area (Huang et al., 2018;Stolzenburg et al., 2018).
Once emitted to the atmosphere, α-pinene is oxidized through reactions with atmospheric oxidants such as ozone (O3),
hydroxyl (OH.) and nitrate (NO3.) radicals. These reactions have been shown to result in numerous oxidation products with
various chemical functionalities, including alcohols, aldehydes, ketones, carboxylic acids, peroxides and peroxy-acids. As a 55
result of their lower volatilities, many α-pinene derived oxidation products (e.g. carboxylic acids) partition to already existing
particles in the atmosphere contributing to particle growth and increased SOA mass. The extent to which an organic compound
undergoes partitioning is related to its saturation vapor pressure and to the available particle mass (Kroll and Seinfeld, 2008).
Due to the temperature dependence of the saturation vapor pressures of organic oxidation products, higher SOA mass yields
from increased condensation of organics are observed at lower temperatures (Pathak et al., 2007;Saathoff et al., 2009;Warren 60
et al., 2009;Svendby et al., 2008).
The saturation vapor pressure of a given organic species is closely related to its chemical structure. Depending on their specific
molecular structure and the amount of strong hydrogen bond donor-acceptor groups, the products of α-pinene oxidation may
undergo condensation or, if sufficiently low volatile, nucleation, with the latter resulting in new particle formation (Kulmala
et al., 2013;Riccobono et al., 2014). Multiple studies report on new particle formation arising from the oxidation of α-pinene 65
and suggest the formation of so-called extremely low volatile organic compounds (ELVOC, Donahue et al. (2012b)) capable
of gas-to-particle conversion (Bonn and Moorgat, 2002;Lee and Kamens, 2005;Gao et al., 2010;Tolocka et al., 2006;Claeys et
al., 2009;Ehn et al., 2014;Metzger et al., 2010;Kirkby et al., 2016;Bianchi et al., 2019). Although the chemical composition of
α-pinene SOA has been extensively studied, uncertainty still remains regarding the chemical structure and functional groups
of the compounds responsible for nucleation. Recent quantum chemical results indicate that multi carboxylic acids with three 70
or more acid moieties are some of the most likely compounds to participate in new particle formation (Elm et al., 2017; Elm
et al., 2019).
Recently, highly oxidized multifunctional organic molecules (HOMs) have been identified in α-pinene oxidation studies
(Bianchi et al., 2019;Ehn et al., 2014;Jokinen et al., 2014;Rissanen et al., 2015). Formed via a gas-phase autoxidation 75
mechanism (Crounse et al., 2013) involving intramolecular hydrogen abstraction by peroxy-radicals, HOMs represent a class
of organic compounds which can promptly reach high degrees of oxygenation (Ehn et al., 2017). Oxygen-to-carbon (O:C)
ratios exceeding unity have been reported for monomeric compounds (Mutzel et al., 2015;Ehn et al., 2014). The high O:C
ratios are attributed to multiple hydroperoxide functionalities and the compounds have been perceived as likely candidates for
nucleation and initial particle growth owing to their low volatilities (Tröstl et al., 2016;Kirkby et al., 2016). However, 80
computational studies have shown that HOMs originating from α-pinene autoxidation can have surprisingly high vapor
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
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pressures considering their O:C ratios, and it is mainly the dimeric compounds that are likely to be classified as ELVOC
(Kurtén et al., 2016;Peräkylä et al., 2019).
High molecular weight (MW > 300 Da) dimer esters formed from oxidation of α-pinene have been identified in laboratory-
generated and ambient air SOA (Hoffmann et al., 1998;Tolocka et al., 2004;Gao et al., 2004;Yasmeen et al., 2010;Witkowski 85
and Gierczak, 2014;Kourtchev et al., 2015;Zhang et al., 2015;Kristensen et al., 2013;Kristensen et al., 2014;Kristensen et al.,
2016;Kristensen et al., 2017;Mohr et al., 2017). Several mechanisms for the formation of dimer esters have been proposed,
including gas-phase mechanisms such as clustering of carboxylic acids (Hoffmann et al., 1998;Claeys et al., 2009;DePalma et
al., 2013) and reactions involving reactive intermediates such as stabilized Criegee intermediates (SCI) and RO2 species
(Kristensen et al., 2016;Zhang et al., 2015;Berndt et al., 2016;Witkowski and Gierczak, 2014;Zhao et al., 2015;Kahnt et al., 90
2018).
The high abundance of dimer esters detected in freshly formed α-pinene derived SOA particles (Kristensen et al., 2016)
suggests that these compounds may be important for the initial formation and growth of atmospheric particles; a hypothesis
supported by saturation vapor pressure estimates reported in Kristensen et al. (2017) classifying dimer esters as ELVOC with
vapor pressures suitable for gas-to-particle phase conversion at room temperatures. Furthermore, recent studies show that 95
dimer esters and other oligomeric compounds in atmospheric aerosol are strongly correlated with cloud condensation nuclei
activity thus suggesting a significant impact on climate (Kourtchev et al., 2016).
In contrast to dimer esters, few studies have identified particle phase HOMs and thus their fate upon gas-particle partitioning
remains elusive. Recently, Zhang et al. (2017) identified highly oxidized monomers (C8-10H12-18O4-9) and dimers (C16-20H24-
36O8-14) in α-pinene derived SOA particles using Na+ attachment during electrospray ionization. The dimers identified by Zhang 100
et al. (2017) show a somewhat different chemical formula and degree of oxidation than the gas-phase HOM dimers previously
identified by Ehn et al. (2014), where the observed dimer composition was C19-20H28-32O11-18. This can be attributed to
decomposition of the hydroperoxides functionality of HOMs from processes such as photolysis, thermolysis and solvation,
yielding alkoxy radicals, esters, and other moieties. The chemical formulas of the dimeric compounds in Zhang et al. (2017)
show good resemblance to the dimer esters published in Kristensen et al. (2016), characterized as C15-19H24-30O5-10. This raises 105
the important question, how different types of identified dimers, including HOMs and dimer esters, are related via gas-particle
partitioning, chemical reactions or other processes.
The Aarhus Chamber Campaign on HOMs and Aerosols (ACCHA) presented in this work was designed s to elucidate the
formation of HOMs and dimer esters from the dark ozonolysis of α-pinene at different temperatures. The effect of temperature
and α-pinene concentration on SOA formation and composition on the formation of HOMs and dimer esters is investigated 110
through a series of chamber experiments at temperatures prevailing at the latitudes of the boreal forests (Hakola et al., 2009)
i.e. from 20 to -15 °C.
The results of ACCHA are presented in multiple publications. The elemental composition of the bulk α-pinene SOA by High
Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) is reported in the companion paper by Jensen et al
(2019). A study presenting factor analysis of PTR-ToF-MS data includes a case study from the ACCHA campaign (Rosati et al., 115
2019). The formation of gas-phase HOMs as measured by nitric acid-based Chemical Ionization Atmospheric Pressure interface
Time of Flight (CI-APi-ToF) mass spectrometer is presented in the work by Quéléver et al. (2019).
In the current work, we present the effect of temperature and α-pinene concentrations on the formation and molecular
composition of SOA particles formed from ozone-initiated oxidation of α-pinene. Specifically, we investigate the contributions
of organic acids and dimer esters to α-pinene SOA formed at temeratures of 20, 0, and -15 °C and examine the changes in 120
molecular composition arising from heating and cooling of SOA particles.
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
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2. Method
2.1 Chamber experiments
Dark ozonolysis of α-pinene was conducted in the Aarhus University Research on Aerosol (AURA) smog chamber. The chamber
is described in detail in Kristensen et al. (2017). In short, the AURA chamber consists of a 5 m3 Teflon bag situated in a 27 m3 125
temperature controlled cold room. The chamber temperature can be varied from -16 °C to +26 °C. The temperature and relative
humidity are monitored in the center of the Teflon bag by a HC02 probe coupled to a HygroFlex HF320 transmitter (Rotronic
AG, Switzerland). In the current study, the chamber was operated using dry purified air (active carbon, Hepa) at atmospheric
pressure. Ozone (O3, ~100 ppb) was added to the chamber using an ozone generator (Model 610, Jelight Company, Inc.) and the
concentration of O3 and oxides of nitrogen (NOx) were monitored by UV photometric (O342 Module, Environment S.A.) and 130
chemiluminescent monitors (AC32M, Environment S.A.), respectively. A known amount of VOC was added to a 10 mL glass
manifold, evaporated and transferred to the chamber through a stainless-steel inlet (Diameter = 10 mm, Length = 100 cm) using heated
(70 °C) N2-flow. The concentration of the added VOC was monitored using a Gas Chromatograph with Flame Ionization Detector
(Agilent 7820A GC-FID, with a time resolution of 6 min) and a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-
ToF- 783 MS 8000; IONICON, Innsbruck, Austria) sampling through stainless steel outlets located opposite of the VOC inlet (distance 135
between inlet and outlet ~ 1.6 m). Particle size distributions were measured using a scanning mobility particle sizer (SMPS)
system including a Kr-85 neutralizer (TSI 3077A) and an electrostatic classifier (TSI 3082) coupled with a nano water-based
condensation particle counter (CPC, TSI 3788). The SMPS system was optimized for measurements of particles in the range
of 10 – 400 nm with a sampling time of 80 s (60 s upscan, 20 s downscan, aerosol flow rate = 1.6 L min-1 , sheath flow rate =
5 L min-1). In addition, the total particle number concentration of particles with diameter (Dp) larger than ~1.4 nm was measured 140
by a Particle Size Magnifier (PSM A10, Airmodus, (Vanhanen et al., 2011), sample flow = 2.5 L min-1, time resolution = 1 s)
operated in fixed saturator flow mode.
The chemical composition of gas-phase HOMs were measured using a nitric acid-based Chemical Ionization Atmospheric Pressure
interface Time-of-Flight mass spectrometer (CI-APi-ToF, Tofwerk A.G., Switzerland / Aerodyne Research Inc., USA) presented by
Junninen et al. (2010) and Jokinen et al. (2012). An Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-145
AMS, Aerodyne Research Inc., (DeCarlo et al., 2006)) was deployed to measure real-time, non-refractory particulate matter. In
addition, SOA molecular composition with respect to organic acids and high-molecular-weight dimer esters was investigated through
off-line filter analysis of the formed α-pinene derived SOA performed using an Ultra High-Performance Liquid Chromatograph
coupled to the electrospray ionization source of a Bruker Daltonics quadrupole time-of-flight mass spectrometer (UHPLC/ESI-qToF-
MS). Filter sampling was performed once no additional growth in SOA mass was evident from the SMPS and no α-pinene could be 150
detected by the GC-FID. All filter samples were collected by a low volume sampler onto 47 mm 0.20 micrometer PTFE filters
(Advantec) situated in stainless steel filter holders at a flow rate of ~27 L min−1 once no additional growth in SOA mass was evident
from the SMPS and no α-pinene could be detected by the GC-FID. After collection, the filter samples were stored at -20 °C until
extraction and analysis.
A total of 12 α-pinene oxidation experiments conducted in the AURA smog chamber are presented here (Table 1). Each 155
experiment was performed at temperatures close to either 20, 0 or -15 °C to investigate the effect of temperature on the
formation and composition of both gas and particle phase organics from dark ozonolysis of α-pinene. Five experiments were
conducted with the injection of 10 ppb α-pinene (~0.32 μL, 99%, Sigma Aldrich, Exp. 1.1-1.5) into the ozone-filled chamber
(~100 ppb O3). These experiments include oxidation at constant temperatures 20, 0 and -15 °C (Exp. 1.1, 1.2 and 1.3) and two
temperature ramp experiments (Exp. 1.4 and 1.5). The ramp experiments were performed by injection of α-pinene at a fixed 160
20 °C (Exp. 1.4) or -15 °C (Exp. 1.5) temperature followed by subsequent ramping to -15 or 20 °C, respectively. In both
experiments the temperature ramping was initiated approximately 40 min after the injection of α-pinene. In Exp. 1.4 a decrease
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
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in temperature from 20 °C to -15 °C was achieved in ~ 100 min, while in Exp. 1.5 heating of the chamber from -15 °C to 20
°C was performed in ~ 140 min.
To investigate the effect of α-pinene concentration on the SOA formation and composition three experiments were performed 165
with 50 ppb of α-pinene (~1.6 μL, 99%, Sigma Aldrich) at 20, 0 and -15 °C (Exp. 2.1-2.3).
The injection of α-pinene in the 10 and 50 ppb experiments described above (Exp. 1.1-2.3) was conducted with a N2 flow rate
of 15 L min-1. To examine the effect of the injection flow rate, the 50 ppb α-pinene Exp. 2.1-2.3 were repeated using an N2injection flow of 30 L min-1 (Exp. 3.1-3.3).
Throughout this paper SOA density is assumed to be 1 g cm-3 as in Kristensen et al. (2017). 170
2.2 Off-line particle analysis by UHPLC/ESI-qTOF-MS
Collected particle filter samples were extracted and analyzed for organic acids and dimer esters. Filter extraction was
performed using a 1:1 mixture of methanol and acetonitrile (HPLC grade, Sigma Aldrich). 0.2 μg of camphoric acid recovery
standard was added to the filter sample prior to the addition of the extraction mixture. The samples were placed in a cooled
ultrasonic bath for 15 min after which the filters were removed and the extracts filtered through a Teflon filter (0.45 µm pore 175
size, Chromafil). The extraction solvents were then removed by evaporation over gentle N2 flow and the residues were
dissolved by adding 0.2 mL MilliQ water with 10 % acetonitrile and 0.1 % acetic acid. The reconstituted samples were placed
in a cooled ultrasonic bath for 5 min and the extract transferred to a HPLC sample vial for analysis. To ensure complete transfer
of organics from the evaporated extracts, additional reconstitution were performed with 0.2 mL MilliQ water with 50 %
acetonitrile and 0.1 % acetic acid. Both sample solutions (10 % and 50 % acetonitrile reconstitution) were analyzed by 180
UHPLC/ESI-qTOF-MS immediately after extraction.
The HPLC stationary phase was a Waters Acquity UPLC Ethylene Bridged Hybrid C18 column (2.1 x 100 mm 1.7 um) while
the mobile phase consisted of acetic acid 0.1% (v/v) in MilliQ water (eluent A) and acetic acid 0.1% (v/v) in acetonitrile as
eluent B. The operating conditions of the mass spectrometer have been described elsewhere (Kristensen and Glasius, 2011).
Quantification of the organic acids was performed from eight-level calibration curves (0.1 to 10.0 μg mL-1) of the following 185
acids: cis-pinic, cis-pinonic acid, terpenylic acid, diaterpenylic acid acetate (DTAA), 3-methyl-butane tricarboxylic acid
(MBTCA). Due to lack of authentic standards, the dimer esters were quantified using DTAA as surrogate standard.
3. Results and discussions
A representative example of the results obtained from the conducted smog chamber experiments is shown in Fig. 1. Following
injection of α-pinene into the ozone-filled chamber at constant temperature and RH, rapid particle formation is captured by the 190
PSM (measuring particles >1.4 nm) and, shortly after, the SMPS (10-400 nm). In all conducted experiments, maximum particle
number concentrations were obtained within 10 minutes after α-pinene injection followed by decay ascribed to wall loss and
particle aggregation. Particle number concentrations shown in Fig. 1 (lower panel) are wall-loss corrected. During the course
of the experiment, decrease in ozone and α-pinene concentration is accompanied by increase in particle SOA mass
concentration as measured by the SMPS. Figure 1 (upper panel) shows wall-loss-corrected SOA mass concentration obtained 195
from ozone-initiated oxidation of 50 ppb α-pinene (measured by GC-FID). Wall loss corrections were based on first order fits
to the SOA mass or number concentration after peak. The grey shaded area represents the time of filter sampling for off-line
chemical analysis by UHPLC/ESI-qTOF-MS. Similar figures showing data from all conducted experiments are presented in
Supplementary Information.
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3.1 Particle formation rates and SOA mass yields 200
Figure 2A-B show the total particle number concentration (# cm-3) as measured by the PSM (Dp> 1.4 nm ) at 20, 0 and -15 °C
as a function of time after injection of 10 ppb and 50 ppb α-pinene, respectively. The maximum total particle number
concentrations in all experiments are listed in Table 1. Particle formation in terms of number concentrations from the dark
ozonolysis of α-pinene at both 10 ppb and 50 ppb VOC concentrations show negative dependence on temperature as has also
been reported previously (Jonsson et al., 2008;Kristensen et al., 2017). Maximum particle formation rates (# cm-3 s-1) are 205
estimated from linear fits to the experimental time series of total particle number concentration measured by the PSM (Fig.
2A-B inserts). It is clear that lower temperatures result in higher particle formation rates from ozone-initiated oxidation of α-
pinene at both low (10 ppb) and high (50 ppb) VOC concentration. At both α-pinene concentrations, a significantly higher
particle formation rate is observed when oxidation takes place at 0 °C compared to oxidation performed at 20 °C, with the
formation rates being ~ 6 and ~ 20 times higher at 0 °C than at 20 °C, at 10 ppb and 50 ppb α-pinene experiments, respectively. 210
In comparison, the maximum particle formation rate at both α-pinene concentrations only increased by a factor of 1.5 when
performing the oxidation at -15 °C compared to 0 °C.
Figures 2C-D show the evolution of the wall-loss-corrected SOA mass concentration (µg m-3) measured by SMPS as a function
of time after α-pinene injection. Agreeing with previous studies, SOA mass and mass yields are higher at lower temperatures
(Jonsson et al., 2008;Saathoff et al., 2009;Pathak et al., 2007;Kristensen et al., 2017). The reported 18 and 43 % mass yields 215
at 20 °C and -15 °C, respectively, (50 ppb α-pinene, 100 ppb O3) are in excellent agreement with previous values of 21 % and
39 % from similar oxidation experiments (50 ppb α -pinene, 200 ppb O3) performed in the same chamber (Kristensen et al.,
2017). Compared to experiments performed at 20 °C, temperatures of 0 °C and -15 °C respectively result in ~ 50 % and ~ 150
% higher SOA mass concentrations. Interestingly, SOA mass concentration in both 10 and 50 ppb α-pinene oxidation
experiments show almost identical response to temperature going from reaction temperatures of 20 °C to -15 °C. In Fig. 2C 220
and 2D the maximum SOA mass is obtained faster at higher reaction temperatures than at lower temperatures. For both low
(10 ppb) and high (50 ppb) α-pinene loadings, maximum SOA mass is reached after approximately 140 min, 200 min and 250
min after α-pinene injection at 20, 0 and -15 °C, respectively. This is attributed to the faster reaction of α-pinene with ozone
at higher temperatures, which is also evident from the calculated α-pinene loss rates shown in the insert in Fig. 2D (50 ppb,
GC-FID derived; due to the detection limit of the GC-FID no loss rates could be calculated for 10 ppb α-pinene experiments). 225
Compared to oxidation experiments performed at 20 °C, loss rates of α-pinene were found to be 11 % and 22 % lower at 0 °C
and -15 °C reaction temperature, respectively. These results are in good agreement with temperature dependent reaction rates
of ozone with α-pinene reported by Atkinson et al. (1982) predicting a 16 (± 4) % and 28 (± 6) % lower reaction rate at 0 °C
and -15 °C, respectively, relative to the reaction rate calculated at 20 °C. The discrepancy between measured and literature-
derived values is likely due to the presence of OH radicals in the current study. Despite the lower reaction rate for α-pinene 230
ozonolysis at low temperature, the particle number concentration data, in Fig. 2, show significantly faster particle formation
in cold experiments.
3.2 Particle size distributions and sub-10 nm particles
Figure 3A shows the particle size distributions as recorded by the SMPS after max. SOA mass concentration was reached in
(10 ppb) and high (50 ppb) concentration α-pinene oxidation experiments at 20, 0 and -15 °C. At both α-pinene concentrations, 235
lower reaction temperatures result in larger particles consistent with the higher SOA masses at 0 °C and -15 °C in Fig. 3C and
3D. The time evolutions of the particle size distribution measured over time (Fig. 3B) shows that particles formed at 20 °C are
initially larger than the particles formed at the two lower temperatures (0 °C and -15 °C). However, during the course of the
experiments, particles formed at the lower temperatures grow more rapidly to larger sizes compared to particles formed at 20
°C. This is likely attributable to enhanced condensation of organics onto the formed particles at lower temperatures. 240
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
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Figure 4A shows the total particle number concentration (# cm-3) obtained by the PSM (Dp> 1.4 nm ) and SMPS (Dp in the
range 10-400 nm) respectively as a function of time after α-pinene injection (10 ppb) at both 20 and -15 °C. The differences
in the total particle number concentrations derived by the two instruments corresponds to the number concentrations in the
size range of 1.4 nm to 10 nm. Comparing experiments performed at 20, 0 and -15 °C temperatures in Fig. 4B (10 ppb α-
pinene), it is clear that the presence of sub-10 nm particles is higher at the lower reaction temperatures and that the largest 245
temperature dependence is in the range 20 °C to 0 °C.
According to Fig. 4A, sub-10 nm particles constitute ~10 and 25 % of the total particle number concentration at 20 and -15
°C, respectively, even 30 min after α-pinene injection where particle number concentrations have peaked. The size distributions
obtained from the SMPS (Fig. 4C), however, indicating very few particles in the smaller size-ranges (i.e. below 20 nm, Fig.
4C insert) at this time of the experiments. A possible explanation for the high sub-10 nm particle number concentration is the 250
presence of a particle distribution mode below the detectable range of the SMPS. Supporting this, Lee et al. (2016) reported
an absence of particles in the size range between 3 and 8 nm in ambient air particle measurements (1-600 nm particle size
distribution range combining PSM and SMPS data) during the Southern Oxidant and Aerosol Study (SOAS) field campaign.
To our knowledge, the current work represents the first indication of this observed “gap” in the particle size distribution in
smog chamber experiments, thus emphasizing the need for further studies of sub-10 nm particle formation and detection from 255
VOC oxidation.
3.3 Molecular composition
Figure 5 shows the detailed chemical composition of SOA with respect to organic acids and dimer esters at different
temperatures (20 °C, 0 °C, and -15 °C) and at two different concentrations of α-pinene (10 ppb and 50 ppb, respectively). The
acids and dimer esters identified are similar to those in Kristensen et al. (2017), where suggestions for molecular structures are 260
given. Significantly higher particle concentrations of α-pinene-derived organic acids are evident at the lower 0 and -15 °C
temperatures compared to 20 °C supporting increased condensation of organics at lower temperatures. The total contributions
of the identified acids to the formed SOA mass in experiments with an initial α-pinene concentration of 10 ppb are 17 %, 25
% and 32 % at 20, 0 and -15 °C, respectively. In comparison, in 50 ppb α-pinene oxidation experiments, the acids contribute
to the formed SOA with 18 %, 38 % and 28 % respectively. For comparison, the fraction of acids was 20 % at 20 °C and 31 265
% at -15 °C of total SOA mass in experiments with 50 ppb of α-pinene and 200 ppb of ozone (Kristensen et al. 2017).
Comparing mass fractions of acids at 20 °C and -15 °C, the mass fraction seems to be highly dependent on temperature and
much less dependent on VOC concentration or VOC:O3 ratios.
In all experiments, pinic acid was found to be the dominant identified organic acid, constituting between 5 and 9 % of the
formed SOA mass with highest contributions at lower reaction temperatures. A negative dependence of concentration with 270
temperature is observed for six of the ten identified organic acids: pinonic acid, pinalic acid, oxopinonic acid, OH-pinonic
acid, pinic acid and terpenylic acid. Diaterpenylic acid acetate (DTAA), diaterpenylic acid (DTA) and 3-methyl-
butanetricarboxylic acid (MBTCA), however, show lower concentrations at the lower temperatures compared to at 20 °C. The
effect of temperature seems to correlate with the O:C ratio (i.e. degree of oxidation of the organic acids ) as shown in Fig. 5.
Acids with lower O:C-ratios (O:C < 0.4) are found at higher concentrations at lower temperatures (0 and -15 °C). This can 275
likely be attributed to an enhanced condensation of these species onto the SOA particles at temperatures of 0 and -15 °C. In
contrast, although contributing significantly less to the SOA mass, the more oxidized compounds (O:C > 0.4, DTAA, DTA
and MBTCA) show an opposite trend with respect to temperature (Fig. S3). As MBTCA, DTAA and DTA are hypothesized
to be formed from gas-phase oxidations involving OH-radicals (Szmigielski et al., 2007;Vereecken et al., 2007;Müller et al.,
2012;Kristensen et al., 2014), their lower concentrations at lower reaction temperatures could be explained by (1) changes in 280
reaction pathways or branching of the intermediates of MBTCA, DTAA and DTA, (2) lower gas-phase concentration of the
first generation of oxidized organic compounds due to higher degree of condensation (e.g. MBTCA is formed from gas-phase
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
8
oxidation of pinonic acid), and (3) reduction of OH radical production and hence reduced oxidation by OH-radicals at lower
reaction temperatures (Jonsson et al., 2008;Tillmann et al., 2010). The former (1) is supported by Müller et al. (2012) and
Donahue et al. (2012a) reporting lower particle-phase MBTCA correlating with decreased pinonic acid vapor fractions at lower 285
temperatures. Both explanations are supported by lower O:C ratios of the total α-pinene-derived SOA formed at the lower
temperatures as reported in Jensen et al. (2020).
A total of 30 dimer esters were identified in the collected SOA particle samples, from both 10 and 50 ppb α-pinene ozonolysis
experiments. In the 10 ppb α-pinene experiments, total concentrations of dimer esters of 0.64, 0.51 and 0.46 µg m-3 were found
in SOA particles formed at 20, 0 and -15 °C, respectively, thus showing a small positive dependence on temperature. The 290
corresponding mass fractions were: 11.1 %, 7.5 % and 3.8 %. For particles formed from higher 50 ppb α-pinene concentrations,
the temperature dependence is less obvious with total dimer ester concentration of 3.1, 4.3, and 3.3 µg m-3 at 20, 0, and -15
°C, respectively. The corresponding mass fractions of the dimer esters are: 8.9 %, 8.0 % and 3.9 %, thus similar to fractions
reported for the 10 ppb α-pinene oxidation experiments, thus, as in the case of the organic acids, the mass fraction of dimer
esters seems to be highly dependent on temperature and much less dependent on VOC concentrations or VOC:O3 ratios. This 295
is somewhat contradictory to previously published findings by Kourtchev et al. (2016) showing increased particle fraction of
oligomeric compounds at higher VOC precursor concentrations.
Looking at the concentrations of the (30) individual dimer esters, their formation is affected differently by the reaction
temperature. As an example, the particle-phase concentration of the MW368 dimer ester (pinonyl-pinyl ester, C19H28O7) is
increases at lower temperatures, while the opposite is seen for the MW358 dimer esters (pinyl-diaterpenyl ester, C17H26O8). 300
Figure 6A shows the yields of the identified dimer esters at -15 ° C relative to 20 °C (yield (-15 °C) / yield (20 °C)) as a
function of the number of carbon atoms in the dimer esters. From this, it is clear that the effect of temperature on the formation
of individual dimer esters is compound specific. Of the 30 dimer esters, increased particle concentrations at lower temperatures
is most often related to species with high carbon number (i.e. C19 species). Also, Fig. 6A shows that the temperature effect on
the dimer ester concentration to a large extent depends on the O:C ratio (indicated by color scale) of the individual species. 305
This is highlighted also in Fig. 6B showing a more significant decrease in relative yields of dimer esters with higher oxygen
number (i.e. more oxidized) at the lower -15 °C temperature in both 10 and 50 ppb α-pinene experiments. Accordingly, the
dimer esters are grouped based on their O:C ratios, with the more oxidized dimer esters having an O:C > 0.4. The total
concentrations of the low (< 0.4) and high (> 0.4) O:C dimer esters are shown in Fig. 5B and 5D inserts. Recently, gas-phase
RO2–RO2 reactions followed by particle phase decomposition of the resulting diacyl peroxides have been suggested as a 310
possible mechanism for the formation of dimer esters from α-pinene ozonolysis (Zhang et al., 2015). As the decomposition of
diacyl peroxides and the subsequent formation of dimer esters is suggested to increase at higher temperatures, this could
indicate that the formation of the high O:C dimer esters proceed through the suggested RO2-RO2 reaction pathway, thus
explaining increased concentrations at higher temperatures. Also, it is worth considering that the lower O:C dimer esters (such
as the MW368 pinonyl-pinyl ester) may be composed of diacyl peroxides or acyloxyalkyl hydroperoxides, thus explaining the 315
observed increased concentrations of these species at the lower temperatures owing to the suggested unstable nature of
hydroperoxide-containing compounds (Krapf et al., 2016) .
Thermodynamics also need to be considered as possible explanation for the observed temperature responses of the high and
low O:C dimer esters. As reported in Kristensen et al. (2017), the identified dimer esters span across a wide range of volatilities.
Here, many of the low O:C dimer esters may be sufficiently volatile to allow considerable fractions to exist in the gas phase 320
at high temperature. Consequently, the increased particle-phase concentration observed at the lower temperatures may solely
be attributed to enhanced gas-to-particle phase partitioning of these species. Supporting this, Mohr et al. (2017) identified
dimeric monoterpene oxidation products (C16–20HyO6–9) in both particle and gas phases in ambient air measurements in the
boreal forest in Finland.
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9
3.4 Temperature ramping 325
Normann Jensen et al. (2020) present a detailed analysis on changes in AMS derived elemental chemical composition during
temperature ramps in the ACCHA campaign and show that temperature at which the particles are formed to a large extend
determines the elemental composition also after subsequent heating or cooling by 35 °C. During two temperature ramp
experiments, we also studied how the concentration, mass fraction and molar yields of carboxylic acids and dimer esters as
identified in the off-line analysis varied. The experiments involving temperature ramps were conducted to examine whether 330
the observed effect of temperature on the dimer ester concentration is attributed to gas-to-particle partitioning or
stabilization/decomposition of the hydroperoxide-containing species.
Figure 7A shows the SOA mass formation (wall-loss corrected) from the 10 ppb α-pinene oxidation experiments performed at
constant 20 °C (Exp. 1.1) and -15 °C (Exp. 1.3) and from the two temperature ramping experiments (Exp. 1.4 and 1.5) in which
the chamber temperature was ramped up or down ~ 40 min after injection of α-pinene. In the ~ 40 min following the injection 335
of α-pinene, SOA mass in both ramp experiments show good agreement with the constant temperature experiments performed
at similar starting temperature (Exp. 1.1 and 1.3). Likewise, particle formation rate and maximum particle number
concentration obtained in Exp. 1.4 and 1.5 prior to temperature changes (Table 1) also resembles that of Exp. 1.1 and 1.3. Thus
the initial evolution of the particle formation in Exp. 1.1 (constant 20 °C) and Exp. 1.4 (ramp experiment initiated at 20 °C)
and experiment 1.3 (constant -15°C) and Exp. 1.5 (ramp experiment initiated at -15 °C) are comparable. 340
From Fig. 7A, cooling and heating the chamber imply a quasi-immediate effect on the SOA mass concentration. In Exp. 1.4,
decreasing the temperature results in a maximum SOA mass concentration being ~ 30 % higher than the maximum SOA mass
in Exp. 1.1 performed at a constant 20 °C (8 versus 6 µg m-3). Reversely, heating the chamber from -15 to 20 °C (Exp. 1.5)
results in a ~ 30 % lower SOA mass (10 versus 15 µg m-3) compared to SOA formed and kept at -15 °C (Exp. 1.3). The changes
in particle size distributions as a result of temperature ramping (Fig. 7B) are attributed to evaporation during heating and 345
condensation during cooling. Interestingly, despite the loss or gain of particle mass during temperature ramping the SOA mass
concentration is closer to the experiments performed at the initial temperature rather than the final temperature even after a
temperature ramp of 35 °C. This, in turn, suggests that the formation conditions to a large extent govern the type of SOA that
was formed, rather than the conditions the SOA was later exposed to.
Figure 8 shows the concentrations, mass fractions and molar yields of organic acids and dimer esters in SOA particles from 350
constant temperature oxidation experiments at 20 and -15 °C (Exp. 1.1 and 1.3, respectively) and from the two temperature
ramp experiments (Exp. 1.4 and 1.5). Upon heating SOA particles from -15 to 20 °C, a loss of particle mass (5 µg m-3, Fig. 7)
was observed and is attributed to evaporation of organics. This is supported by a 2.8 µg m-3 lower organic acid concentration
in particles exposed to heating in Exp. 1.5 compared to particles formed at the constant -15 °C (Exp. 1.3), accounting for ~ 60
% of the evaporated organics. These results underline the semi-volatile nature of the identified organic acids (i.e. pinalic acid, 355
pinonic acid, oxopinonic acid, OH-pinonic acid, pinic acid, terpenylic acid and terebic acid) and show that these acids are
readily removed from organic aerosols during heating thus explaining previously reported reduced aerosol mass fraction from
heating α-pinene SOA (Pathak et al., 2007). Figure 8 reveals that heating and cooling of α-pinene-derived SOA particles results
in insignificant or small changes in the concentrations, mass fractions, and molar yields of dimer esters when compared to
experiments performed at constant temperatures. This indicates that the dimer esters are not subjected to evaporation or 360
decomposition within the studied temperature range and timeframe. These results show that the concentration of the dimer
esters in SOA from α-pinene oxidation is largely determined by the temperature by which the SOA is formed rather than
subsequent exposure to higher or lower temperatures.
3.5 Dimer ester formation and HOMs
A detailed study on the formation of HOMs during the ACCHA campaign is presented by Quéléver et al. (2019). Here, 365
significantly lower (by orders of magnitude) HOM gas-phase concentrations are observed in -15 °C experiments compared to
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
10
20 °C experiments. As the HOMs form through autoxidation of RO2, low temperatures and thus decreased autoxidation is
expected to result in lower formation of HOMs, with reduced formation of the more oxygenated species. Interestingly,
however, Quéléver et al. (2019) observed no correlation between the degree of oxidation (i.e. O:C-ratio) of the identified
HOMs and the magnitude by which the formation of these was reduced at lower temperatures. This is in contrast to the 370
observed temperature effects on the formation of the identified dimer esters presented in the current study (Fig. 7). In Quéléver
et al. (2019), one possible interpretation of the observed temperature effect on HOMs is that the rate-limiting step in the
autoxidation chain takes place in RO2 radicals with six or fewer oxygen atoms. This is supported by the observed decreased
concentration of dimer esters with a higher number of oxygen atoms, which also indicates that the formation of the identified
dimer esters likely proceeds through reaction of RO2. The observed response to temperature of the different dimer esters may 375
thus be ascribed to the oxidation state of RO2 species available for gas-phase reactions. At the lower reaction temperatures, the
RO2 autoxidation is limited, thus favoring the formation of low O:C dimer esters through reaction of less oxidized RO2 species.
Likewise, the higher O:C dimer esters thus only form if sufficient degree of autoxidation of the RO2 is allowed before
bimolecular cross-reaction takes place and the dimer esters are formed.
We thus propose that, although different in chemical structures and O:C-ratios, dimer esters and HOMs may be linked through 380
their formation mechanisms, both involving RO2 autoxidation. The particle-phase dimer esters and the gas-phase HOMs may
merely represent two different fates of the RO2 radicals. If conditions are favorable and efficient autoxidation takes place, this
will result in the formation of HOMs, which by the definition recommended by Bianchi et al. (2019) in this case means any
molecule with 6 or more oxygen atoms that has undergone autoxidation. On the other hand, dimer esters could be the product
of RO2 cross reactions, with O:C ratios influenced by the number of potential autoxidation steps undertaken by the involved 385
RO2 species prior to reaction (Fig. 9). In this case, if either RO2 has undergone autoxidation, the resulting dimers would classify
as HOM, regardless whether the ultimate fate is a dimer peroxide or dimer ester. The formation of dimer esters through
reactions of RO2 is supported by an observed decrease in dimer esters concentrations at higher levels of NOx in ambient air
measurement in Hyytiälä, Finland (Kristensen et al., 2016) and supports the formation proposed by several studies (Ehn et al.,
2014;Berndt et al., 2018;Zhao et al., 2018) involving RO2 cross-reactions as likely route of gaseous dimer formation. 390
4. Conclusions
The formation of SOA from dark ozonolysis of α-pinene is highly influenced by temperature. At sub-zero temperatures, such
as -15°C, more effective nucleation gives rise to significantly higher particle number concentration compared to similar
experiments performed at 20 °C, where the vast majority of laboratory studies are conducted. In addition, the SOA mass
concentration resulting from α-pinene ozonolysis shows a strong temperature dependence attributed to increased condensation 395
of semi-volatile oxidation products at lower temperatures. This is supported by higher concentration of semi-volatile organic
acids, such as pinic acid and pinonic acid, in SOA particles formed at 0 °C and -15 °C compared to particles formed at 20 °C.
In addition to organic acids, the contribution of high-molecular-weight dimer esters to the formed SOA is also affected by
temperature. Underlining the chemical complexity of α-pinene SOA, the 30 quantified dimer esters showed different behaviors
with respect to temperature, with the suppressed formation of the more oxidized dimer esters (O:C > 0.4) at low reaction 400
temperatures. This feature is not seen in the case of the least oxidized dimer esters (O:C < 0.4), showing a small increase in
particle concentration at lower temperatures. Similar to the high O:C dimer ester compounds, α-pinene ozonolysis experiments
performed at lower temperatures result in lower HOM formation in the gas-phase. We suggest that the identified dimer esters
may form through RO2 cross-reactions likely involving hydroperoxide-containing compounds from autoxidation. The
identification of dimer esters in SOA would thus signify a specific termination pathway and fate of the RO2 radicals, and link 405
these molecules more close to HOMs than earlier thought. These results indicate that temperature not only affects the formation
of SOA mass in the atmosphere but also alters the chemical composition through condensation and evaporation of semi-volatile
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
11
species, changes in the formation of HOMs and finally in the reaction pathways leading to the formation of dimer esters having
high and low O:C-ratios.
With respect to dimer esters, no decomposition or evidence of partitioning changes between the aerosol and the gas phase were 410
observed from heating or cooling the SOA particles, suggesting that 1) the formation and, consequently, concentration of dimer
esters is dictated by the VOC oxidation conditions and 2) once formed, dimer esters remain in the particle phase, representing
a core compound within SOA and thus an efficient organic carbon binder to the aerosol phase. In relation, the presented results
from temperature ramping show that final SOA mass concentration obtained from dark ozonolysis of α-pinene is more
dependent on the initial reaction temperatures rather than temperatures to which the formed SOA is subsequently exposed. In 415
conclusion, this means that the changes at ambient temperatures in areas in which emissions and oxidation of VOCs, such as
α-pinene, is likely to result in significant changes to the resulting SOA mass as well as the chemical composition of the SOA.
As global temperatures are expected to rise, especially in the Nordic regions of the boreal forests, this means that although less
SOA mass is expected to form from the oxidation of emitted biogenic VOCs, the temperature-induced changes to the chemical
composition may result in more temperature resistant SOA influencing cloud-forming abilities from increased content of 420
oligomeric compounds (i.e. dimer esters).
Author contributions
MB, ME, and MG and HBP supervised the ACCHA campaign. KK, LLJQ, SC, ME, and MB designed the experiments. KK,
LNJ, SC initialized the chamber for experiments. KK and LNJ measured and analyzed the aerosol phase. KK, BR, and RT
measured and analyzed the VOCs and their oxidation production. LLJQ performed the measurement and analyzed the gas-425
phase HOMs. JE guided and helped with the interpretation of the dimer ester data and formation pathways. KK prepared the
manuscript with the contributions from all co-authors.
Acknowledgement
We thank Aarhus University and Aarhus University Research Foundation for support. This work was supported by the 430
European Research Council (Grant 638703-COALA ) and the Academy of Finland (grants 307331, 317380 and 320094). KK
acknowledge the Carlsberg Foundation (Grant CF18-0883) for financial support. JE thanks the Villum foundation and the
Swedish Research Council Formas project number 2018-01745-COBACCA for financial support.
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12
435
Table 1. Overview of conducted α-pinene ozonolysis experiments
440 a Temperature and RH measured in centre of Teflon bag, b Average temperature and RH (±std.dev.) over the entire experiment, c VOC loss rates are estimated from GC-FID measurements d Measured by SMPS (10-400nm), e Measured by PSM (>1.4nm),
* PSM overloaded 445 ∞ Temperature ramps were performed after the stable temperature phase and is presented in Jensen et al., (2019).
ID Date Exp. Type
α-pinene (ppb)
α-pinene injection flow
(LPM)
Ozone (at injection)
(ppb)
Temp. a (at injection)
(°C)
RH a (at injection)
(%)
Temp. avg. b (°C)
RH avg. b
(%)
VOC loss rate
c
(h-1)
Max SOA
mass d (µg m-3)
Max particle number d
(10-400nm) (# cm-3)
Max particle number (>1.4nm)
(# cm-3) e
1.1 20161202 Low load, 20 °C 10 15 104 20.2 0 20.3
(±0.1) 0.8
(±0.8) N/A 6 3.9∙104 5.2∙104
1.2 20161208 Low load, 0 °C 10 15 105 0.9 2.9 0.7
(±2.7) 7.1
(±4.1) N/A 9 7.9∙104 17.0∙104
1.3 20161207 Low load, -15 °C 10 15 106 -13.7 8 -14.7 (±0.1)
12.9 (±5.3) N/A 15 7.4∙10
4 19.5∙104
1.4 20161209 Low load, 20 to -15 °C 10 15 103 19.9 0 N/A N/A 8 4.4∙104 8.4∙104
1.5 20161220 Low load, -15 to 20 °C 10 15 113 -14 11.7 5.3
(±4.0) N/A 10 9.4∙104 20.0∙104
2.1∞ 20161212 High load, 20 °C 50 15 105 19.8 0 20
(±1.1) 1.1
(±1.3) 1.0 50 8.2∙104 11.0∙104
2.2 20161219 High load, 0 °C 50 15 107 -0.4 6.9 -0.3
(±0.1)6.9
(±0.8) 0.9 65 19.0∙104 >70.0∙104 *
2.3a 20161213 High load, -15 °C 50 15 101 -13.1 6.7 -14.9 (±0.6)
11.9 (±4.3) 0.8 120 24.0∙10
4 >70.0∙104 *
2.3b∞ 20161221 High load, -15 °C 50 15 113 -14.0 19.8 -15.0 (±0.2)
24.7 (±3.6) 0.8 131 16.0∙10
4 51.2∙104
3.1 20170112 High load, 20 °C 50 30 100 20.3 0 20.1
(±0.5)2.4
(±2.0) 1.1 50 7.8∙104 8.4∙104
3.2 20170116 High load, 0 °C 50 30 105 0.2 8.6 0.0
(±0.1)8.7
(±1.1) 1.0 78 25.0∙104 >70.0∙104 *
3.3 20170113 High load, -15 °C 50 30 105 -14.5 11.2 -14.8 (±0.6)
15.8 (±4.5) 0.9 115 23.0∙10
4 >70.0∙104 *
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13
Figure 1. Concentration of O3 (ppb, black), α-pinene (ppb, green), wall-loss corrected SOA mass (µg m-3, red) and particle 450
number concentration (# cm-3, measured by PSM (>1.4nm) and SMPS (10-400nm), dark red) and the geometric mean particle
size (nm, by SMPS, violet) along with recorded RH (%, teal) and temperature (°C, blue) during a 50 ppb α-pinene oxidation
experiment performed at 20 °C (Exp. 2.1).
120
100
80
60
40
20
0α-p
inen
e (p
pb),
Ozo
ne (p
pb)
2001751501251007550250Exp. Time (min after α-pinene injection)
25 2520 2015 1510 10
5 50 0
Tem
p (°
C),
RH
(%)
1.2 x105
1.00.80.60.40.20.0Pa
rticl
e #
(cm
-3)
70
60
50
40
30
20
10
0
SOA m
ass (µg SOA m
-3)
120
80
40
Particle size (nm)
Temp
RH
SOAO3
>1.4 nm
Particle size
Filte
r sam
plin
gα-pinene
10-400nm
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14
455
Figure 2. Effect of reaction temperature and initial VOC loading on SOA formation: Particle number concentration (A, B)
and wall-loss-corrected SOA mass concentration (C, D) at α-pinene concentrations of 10 ppb (A, C) and 50 ppb (B, D). Inserts
show particle formation rates (# cm-3 s-1) as estimated from linear fits to the experimental data (A, B). The insert in D shows
the loss rate of α-pinene (h-1) at different temperatures as derived from an exponential fit to the measured concentration of α-
pinene. 460
Figure 3. A) Particle size distribution recorded 180 minutes after the ozone-initiated oxidation of 10 (Exp. 1.1-1.3) and 50 ppb (Exp. 2.1-2.3) α-pinene performed at 20 °C (red), 0 °C (orange), and -15 °C (blue). B) Particle size distribution recorded
at 10, 30 and 180 minutes after injection of 50 ppb α-pinene at 20 °C (Exp. 2.1, red), 0 °C (Exp. 2.2, orange), and -15 °C (Exp.
2.3, blue). 465
700 x103
600
500
400
300
200
100
0
Parti
cle
(>1.
4nm
) num
ber c
onc.
(#
cm
-3)
400 10 20 30
>700.000 cm-3
>700.000 cm-3
78.000 cm-3
12 x103
10
8
6
4
2
0
1
20°C 0°C -15°C
200 x103
150
100
50
0
Parti
cle
(>1.
4nm
) num
ber c
onc.
(#
cm
-3)
400 10 20 30
195.000 cm-3
170.000 cm-3
52.000 cm-3
20°C 0°C-15°C
1000
800
600
400
200
0
1
20°C 0°C -15°C
Max particle formation rate (# cm-3 s-1)
Max particle formation rate (# cm-3 s-1)
120
100
80
60
40
20
0
SOA
mas
s (µ
g m
-3)
4000 100 200 300
120 µg m-3, Yield = 43 %
78 µg m-3, Yield = 28 %
50 µg m-3, Yield = 18 %
20°C 0°C-15°C15
10
5
0
SOA
mas
s (µ
g m
-3)
2000 50 100 150
15 µg m-3
, Yield = 27 %
9 µg m-3, Yield = 16 %
6 µg m-3, Yield = 11 %
20°C 0°C-15°C
1.0
0.9
0.8
0.7
0.6
0.5
1
20°C 0°C -15°C
α-pinene loss rate (h-1
)
6 x105
5
4
3
2
1
0
dN/d
logD
p (#
cm
-3)
2 3 4 5 6 7 8 9100
2 3 4 5
Dp (nm)
20°C, 10 min 0°C, 10 min-15°C 10 min
20°C, 30 min 0°C, 30 min-15°C, 30 min
20°C, 180 min 0°C, 180 min-15°C, 180 min
2.0 x105
1.5
1.0
0.5
0.0
dN/d
logD
p (#
cm
-3)
2 3 4 5 6 7 8 9100
2 3 4 5
Dp (nm)
10 ppb, 20°C, 180min 10 ppb, 0°C, 180min 10 ppb, -15°C, 180min 50 ppb, 20°C, 180min 50 ppb, 0°C, 180min 50 ppb, -15°C, 180min
A
10 ppb α-pinene 50 ppb α-pinene
Time after α-pinene injection (min) Time after α-pinene injection (min)
Time after α-pinene injection (min) Time after α-pinene injection (min)
B50 ppb α-pinene
A B
C D
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15
Figure 4. A) Effect of temperature on particle number concentrations (# cm-3) as derived from the SMPS (10-400 nm particle size range, broken line) and PSM (>1.4 nm particle size range, solid line) in experiments with an initial α-pinene concentration
of 10 ppb. (Exp. 1.1, red, and Exp. 1.3, blue). B) Particle number concentrations (# cm-3) of 1.4 – 10 nm particles in experiments
with an initial α-pinene concentration of 10 ppb performed at -15°, 0 °C and 20 °C (Exp. 1.1, 1.2, and 1.3, respectively). C) 470
Particle size distributions recorded 30 minutes after the ozone-initiated oxidation of 10 ppb α-pinene performed at 20 °C (red),
0 °C (orange), and -15 °C (blue) (Exp. 1.1-1.3).
Figure 5. LC-MS results showing concentrations (µg m-3, left axis) of acids and dimer etsers as well as O:C ratios (grey bars,
right axis) of these at 20 °C (red), 0 °C (orange), and -15 °C (blue) for the two α-pinene loadings; 10 ppb (top panels A and 475
B), 50 ppb (bottom panels C and D). Inserts show the total concentrations of the identified organic acids and dimer esters (O:C
< 0.4 and O:C > 0.4)
5 x104
43210
2018161412Dp (nm)
1.4 x105
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Parti
cle
num
ber c
onc.
(# c
m-3
)
50403020100Time after α-pinene injection (min)
1.4 - 10 nm -15ºC 1.4 - 10 nm 0ºC 1.4 - 10 nm 20ºC
2.0 x105
1.5
1.0
0.5
0.0
Parti
cle
num
ber c
onc.
(# c
m-3
)
50403020100Time after α-pinene injection (min)
PSM (>1.4 nm) -15ºC PSM (>1.4 nm) 20ºC SMPS (10-400 nm) -15ºC SMPS (10-400 nm) 20ºC 1.4 - 10 nm -15ºC 1.4 - 10 nm 20ºC
2.0 x105
1.5
1.0
0.5
0.0
dN/d
logD
p (#
cm
-3)
2 3 4 5 6100
2 3 4 5
Dp (nm)
20ºC, 30 min 0ºC, 30 min-15ºC, 30 min
0.6
0.5
0.4
0.3
0.2
O:C
0.20
0.15
0.10
0.05
0.00
O:C < 0.4 Dimer Esters
20 °C 0 °C-15 °C
O:C
O:C > 0.4 Dimer Esters
0.6
0.5
0.4
0.3
0.2
O:C
1.0
0.8
0.6
0.4
0.2
0.0
C19
H28
O5
(MW
336
)C
19H
30O
5 (M
W 3
38)
C17
H26
O5
(MW
310
)C
19H
28O
6 (M
W 3
52)
C19
H30
O6
(MW
354
)C
18H
28O
6 (M
W 3
40)
C19
H28
O7
(MW
368
)C
19H
30O
7 (M
W 3
70)
C16
H24
O6
(MW
312
)C
16H
26O
6 (M
W 3
14)
C18
H28
O7
(MW
356
)C
15H
26O
6 (M
W 3
02)
C17
H26
O7
(MW
342
)C
17H
28O
7 (M
W 3
44)
C19
H28
O8
(MW
384
)C
19H
30O
8 (M
W 3
86)
C16
H24
O7
(MW
328
)C
16H
26O
7 (M
W 3
30)
C18
H28
O8
(MW
372
)C
15H
24O
7 (M
W 3
16)
C17
H26
O8
(MW
358
)C
17H
28O
8 (M
W 3
60)
C19
H28
O9
(MW
400
)C
16H
24O
8 (M
W 3
44)
C18
H28
O9
(MW
388
)C
17H
26O
9 (M
W 3
74)
C15
H24
O8
(MW
332
)C
18H
30O
10 (M
W 4
06)
C16
H26
O9
(MW
362
)C
16H
26O
10 (M
W 3
78)
O:C < 0.4 Dimer Esters
20 °C 0 °C-15 °C
O:C
O:C > 0.4 Dimer Esters0.7
0.6
0.5
0.4
0.3
0.2
10
8
6
4
2
0
C10
H16
O3
(Pin
onic
aci
d)C
9H14
O3
(Pin
alic
aci
d)C
10H
14O
4 (O
xopi
noni
c ac
id)
C10
H16
O4
(OH
-pin
onic
aci
d)C
9H14
O4
(Pin
ic a
cid)
C8H
12O
4 (T
erpe
nylic
aci
d)C
7H10
O4
(Ter
ebic
aci
d)C
10H
16O
6 (D
TAA)
C8H
14O
5 (D
TA)
C8H
12O
6 (M
BTC
A)
Org. acids
20 °C 0 °C-15 °C O:C
0.7
0.6
0.5
0.4
0.3
0.2
1.61.41.21.00.80.60.40.20.0
Org. acids
20 °C 0 °C-15 °C
O:C
4
3
2
1
0Org. acids
B
25
20
15
10
5
0Org. acids
D 3
2
1
0O:C0.4
Dimer esters
0.4
0.3
0.2
0.1
0.0O:C0.4
Dimer esters
Con
cent
ratio
n (μ
g m
-3)
Con
cent
ratio
n (μ
g m
-3)
A B C
A
C
10 p
pb α-pinene
50 p
pb α-pinene
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
16
Figure 6. Comparison of relative yields (yield at -15 °C / yields at 20 °C) for specific dimer esters as a function of dimer ester 480
carbon number (A) and oxygen number (B) in 10 and 50 ppb α-pinene ozonolysis experiments. Color scale indicates the O:C
ratio of the dimer esters.
Figure 7. Effect of ramping temperature (20 °C → -15 °C, Exp. 1.4, and -15 °C → 20 °C, Exp. 1.5) on the wall-loss corrected SOA mass concentration (A) and final particle size distribution (B) from ozonolysis of 10 ppb α-pinene. For comparison, 485
results from constant -15 °C and 20°C temperature experiments (Exp. 1.1 and Exp. 1.3, respectively) are also shown.
2.5
2.0
1.5
1.0
0.5
0.0
Dim
er e
ster
Yi
eld
-15
°C /
Yiel
d 20
°C1098765
Dimer ester oxygen number
10 ppb α-pinene50 ppb α-pinene
B2.5
2.0
1.5
1.0
0.5
0.0
Dim
er e
ster
Yi
eld
-15
°C /
Yiel
d 20
°C
1918171615Dimer ester carbon number
0.6
0.5
0.4
0.3
O:C
10 ppb α-pinene50 ppb α-pinene
0.6
0.5
0.4
0.3
O:C
1.2 x105
1.0
0.8
0.6
0.4
0.2
0.0
dN/d
logD
p (#
cm
-3)
2 3 4 5 6 7 8 9100
2 3 4 5
Dp (nm)
20°C-
-
15°C20°C→ -15°C15°C→ 20°C
14
12
10
8
6
4
2
0
SOA
mas
s (μ
g m
-3)
2500 50 100 150 200
Time after α-pinene injection (min)
20
10
0
-10
Temperature (°C
)
-15 °C
-15 °C → 20 °C
20 °C → -15 °C
20 °C
A
A B
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
17
Figure 8. Effect of α-pinene concentration and temperature on the mass concentrations (µg m-3), SOA mass fractions, and molar yield of the identified organic acids (column A), low O:C (0.4) dimer
esters (column C). Note that the concentrations (µg m-3) of organic acids and dimer esters related to the 10 ppb α-pinene 490
oxidation experiments have been multiplied with a factor of 5 (top panels). For the 50 ppb pinene oxidation experiments
average values are reported from Exp. 2.1-2.3 and Exp. 3.1-3.3. Error bars represent one standard deviation.
Figure 9. Illustration of the proposed relation between low O:C (< 0.4) and high O:C (> 0.4) dimer ester formation and HOMs.
6
4
2
0
% m
olar
yie
ld
10 pp
b
50 pp
b
10 pp
b
50 pp
b
10 pp
b
50 pp
b
10 pp
b
10 pp
b
α-pinene concentration
3025201510
50C
once
ntra
tion
(μg
m-3
)
0.4
0.3
0.2
0.1
0.0SOA
Mas
s Fr
actio
n
20 ºC 0 ºC-15 ºC 20 ºC → -15 ºC-15 ºC → 20 ºC
T Ramp
x5 x5
x5
x5 x5
0.5
0.4
0.3
0.2
0.1
0.0
10 pp
b
50 pp
b
10 pp
b
50 pp
b
10 pp
b
50 pp
b
10 pp
b
10 pp
b
α-pinene concentration
4
3
2
1
0
0.06
0.04
0.02
0.00
T Ramp
x5 x5x5
x5
x5
0.3
0.2
0.1
0.0
10 pp
b
50 pp
b
10 pp
b
50 pp
b
10 pp
b
50 pp
b
10 pp
b
10 pp
b
α-pinene concentration
2.0
1.5
1.0
0.5
0.0
0.06
0.04
0.02
0.00
T Ramp x5
x5
x5 x5
x5
Organic acids O:C < 0.4 Dimer esters O:C > 0.4 Dimer estersCBA
https://doi.org/10.5194/acp-2020-99Preprint. Discussion started: 10 February 2020c© Author(s) 2020. CC BY 4.0 License.
18
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